Cell Junctions, Structure, Function, and Regulation
325
integrins to the ECM protein. Adhesion is strengthened by the transmembrane
association of integrins with intracellular proteins that link the adhesion site to actin
stress Fbers within the cell’s cytoplasm.
Hemidesmosome
A specialized cell-ECM junction that mediates very strong adhesion between the basal
layer of stratiFed epithelia and the underlying basement membrane. The
α
6
β
4
integrin
functions at hemidesmosomes as a transmembrane link between the basement
membrane and cytoplasmic proteins that link hemidesmosomes to the intermediate
Flament cytoskeleton.
Integrins
Heterodimeric transmembrane proteins containing one
α
-andone
β
-subunit.
Integrins are generally known for their ability to bind ECM proteins; however, some
integrins bind soluble ligands and/or counter receptors on other cells. Although
integrins are known as adhesion receptors, they also function as signaling receptors
that regulate cell behavior.
Tight Junction
A cell–cell junction that acts as a barrier to the passage of solutes between epithelial and
endothelial cells and serves as a fence to prevent the movement of proteins within the
plasma membrane from passing between the apical and basal surfaces of the cell layer.
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A critical component in the development and homeostasis of higher eukaryotic
organisms is the adhesion of cells to each other and to large insoluble pro-
teins that form the extracellular matrix (ECM). These adhesions are referred to
as cell–cell and cell–matrix junctions respectively. SpeciFc families of cell sur-
face transmembrane receptors connect these junctions with the cell’s cytoskeleton.
Three different cell–cell junctions promote the stable adhesion between cells: tight
junctions formed by the claudin family, and adherens junctions and desmosomes
both mediated by members of the cadherin family. Members of the integrin fam-
ily function at cell–matrix junctions. A subset of integrins also mediate cell–cell
adhesion events that are important for the immune response. In addition to provid-
ing structural linkages important for tissue architecture, cell–matrix and cell–cell
adhesions also activate signaling pathways that act coordinately with signals from
receptors for growth factors, chemokines, cytokines and morphogens to regulate
the cell adhesion, migration, proliferation, survival, and differentiation. In addi-
tion to adhesion receptors themselves, large multiprotein complexes are assembled
at the cytoplasmic face of these junctions by protein interactions initiated at the
intracellular domains of each type of adhesion receptor. Biochemical and cell
culture experiments have provided information on how these protein interactions
and complexes connect adhesion junctions to the cytoskeleton and the cell’s signaling
apparatus. Many components of these adhesion complexes have been identiFed.
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